An organic transistor has many advantages, for example, the organic transistor has high flexibility in constituent materials, fabrication methods, and product form; the organic transistor can be formed to have a large area; the organic transistor has a simple laminated structure thereby simplifying the fabrication process; further, the organic transistor can be formed by using inexpensive fabrication devices. Moreover, as thin films or circuits can be fabricated easily by printing, spin-coating, or dipping, fabrication cost is low compared to a Si-based semiconductor transistor.
When integrating the organic transistors, it is necessary to pattern an organic semiconductor layer. If the organic transistors are integrated with the organic semiconductor layer not patterned, during operation of the organic transistors, an off current increases and power consumption rises. In addition, the large off current also causes cross-talk when pixels are being processed for displaying.
In the Si-based semiconductor transistors, the patterning is performed by photolithography and etching. For the organic transistors, it is noted that inkjet printing and printing by using a dispenser are promising candidates.
For example, Japanese Laid Open Patent Application No. 2004-297011 (below, referred to as “reference 1”) discloses a method of fabricating an organic transistor by appropriately combining the following techniques, specifically, a technique of applying a charge at a specified position onto a surface to be plated, applying a charge of an opposite polarity onto a coating material, and then directing the coating material to the specified position by Coulomb force; a technique of forming a depression at a specified position on a surface to be plated, and applying a coating material to bury the depression; and a technique of evaporating a solvent after applying a coating material to form a pattern, and irradiating a laser beam onto the pattern to shape the pattern.
Further, Japanese Laid Open Patent Application No. 2004-141856 (below, referred to as “reference 2”) discloses a method of forming an indented region on a surface of a substrate, and depositing a liquid material on a specified region adjacent to the indented region for patterning.
In this case, similar to the Si-based semiconductor materials, a photo-resist is applied, and the substrate is exposed and developed to have a desired pattern, thus forming a resist pattern; then etching is performed with this resist pattern as an etching mask, then, the resist is removed. In this way, the organic semiconductor layer can be patterned. However, when a polymer semiconductor material is used, if a photo-resist is applied onto the organic semiconductor layer for patterning, the performance of the transistor is likely to be degraded. This is because the photo-resist is formed from a solution, which is prepared by dissolving a novolac resin having a naphthoquinone-diazide photo-sensitive group into an organic solvent (such as dimethylbenzene, and cellosolve solutions), and the polymer semiconductor material may be adversely influenced by the organic solvent included in the photo resist.
When pentacene or other crystalline molecules are used as the organic semiconductor material, after photolithography, the performance of the transistor may be degraded, although the degradation level depends on specific situations. Further, when stripping the resist after photolithography, the organic semiconductor material may be damaged by the stripping solution (for example, ethylene glycol monobutyl ether, ethylene glycol monoethanolamine, and others), or be damaged by pure water rinse after stripping.
On the other hand, in a printing technique, since a pattern is written directly, material utilization efficiency can be improved greatly. For this reason, with the organic semiconductor layer patterned by printing, it is possible to simplify the fabrication process, improve yield, and reduce fabrication cost. In addition, since a polymer semiconductor solution can be prepared with a polymer semiconductor material soluble to an organic solvent as an organic semiconductor ink, it is possible to pattern the organic semiconductor layer by printing.
However, considering the available precision of printing, it is difficult to form patterns having a dimension less than 50 μm; hence, it is difficult to perform patterning with a high resolution compared to lithography.
In order to solve this problem, it is proposed to reduce the size of liquid drops in printing. However, this is technically difficult, and is anticipated to have limited effects considering stability, ejection clogging, ejection deflection, and so on. Therefore, in order to further improve the patterning resolution, especially the patterning resolution of the organic semiconductor layer, it is necessary to improve the precision of the printing technique.
Japanese Laid Open Patent Application No. 2006-060113 (below, referred to as “reference 3”) discloses a method of forming source/drain electrodes, which method involves an inkjet patterning technique by utilizing a difference of surface energies. Specifically, the surface of a substrate is patterned into a high-surface-energy portion and a low-surface-energy portion, and an electrode is arranged only on the high-surface-energy portion by inkjet printing. By this technique, it is possible to perform patterning with precision of a few μm.
Concerning fabrication of the organic transistor, the following problems should be solved. In an organic transistor including a gate electrode formed by inkjet printing by using Ag ink, considering the precision of printing, the size of the gate electrode cannot be reduced too much. Thus, overlapping of the gate electrode and source and drain electrodes is large, and it strongly influences parasitic capacitance. In addition, compared to a transistor formed of Si-based semiconductor materials, generally, an organic transistor has low mobility and a cutoff frequency is very low, namely, in a low-speed operation driving mode. For this reason, in order to drive the organic transistor to operate in a high speed, it is necessary to improve the mobility or modify the structure of the device. Further, since the gate electrode formed by inkjet printing has low surface flatness, defects occur easily in the gate insulating film, and this reduces the yield. Hence, it is difficult to form a large-area and uniform gate insulating film, and it is difficult to obtain insulating performance as expected.